Dense fluidized granular media in microgravity

Philip Born,Johannes Schmitz,Matthias Sperl

doi:10.1038/s41526-017-0030-z

Philip Born, Johannes Schmitz + Show 1 more

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https://doi.org/10.1038/s41526-017-0030-z

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Journal: NPJ microgravity	Publication Date: Nov 3, 2017
Citations: 8	License type: open-access

Affiliation: German Aerospace Center, University of Cologne

Abstract

Handling and transport of granular media are inevitably governed by the settling of particles. Settling into a dense state is one of the defining characteristics of granular media, among dissipation and absence of thermal agitation. Hence, settling complicates the adaptation of microscopic theories from atomic, molecular, or colloidal media to granular media. It is desirable to provide experiments in which selectively one of the granular characteristics is tuned to test suitable adaptation of a theory. Here we show that gas fluidization of granular media in microgravity is a suitable approach to achieve steady states closer to thermally agitated systems free of settling. We use diffusing-wave spectroscopy to compare the spatial homogeneity and the microscopic dynamics of gas-fluidized granular media on the ground and in drop tower flights with increasing packing densities up to full arrest. The gas fluidization on the ground leads to inhomogeneous states as known from fluidized beds, and partial arrest occurs at packing fractions lower than the full arrested packing. The granular medium in microgravity in contrast attains a homogeneous state with complete mobilization even close to full arrest. Fluidized granular media thus can be studied in microgravity with dynamics and packing fractions not achievable on the ground.

Highlights

Gravitational settling is one of the defining characteristics of granular media, among dissipative collisions and the absence of thermal agitation
The filling fraction Φ and the packing fraction φ within the granular medium seems to be equal in μg
The intensity distributions obtained from the fast count rate traces of the used hardware correlator show deviations from Γdistributions in both cases, which indicates the presence of temporal fluctuations of φloc in the probed volume (Fig. 1b)

Summary

Introduction

Gravitational settling is one of the defining characteristics of granular media, among dissipative collisions and the absence of thermal agitation. The interplay of these effects with the required agitation leads to complex fluidized states of granular media. This situation of too large agitation timescales is characterized by enhanced fluctuations of the local number density or clustering of the granular medium.[8]

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Conclusion